Introduction to Nanotechnology

328 BIOLOGICAL MATERIALS
tions they can adsorb at an air-water interface. Above a certain surfactant
concentration, called the critical micellar concentration, distributions of micelles
in the size range from 2 to lorn can form in equilibrium with free surfactant
molecules, being continuously constituted and disassembled with lifetimes measured
in microseconds or seconds. Synthetic surfactants with more bulky hydrophobic
groups, meaning larger packing parameters, produce extended bilayers that can close
in on themselves to form vesicles that are generally spherical. These structures form
above a critical vesicular concentration. Vesicles typically have lifetimes measured in
weeks or months, so they are much more stable than micelles.
If the vesicles are formed from natural or synthetic phospholipids, they are called
unilaminar or single-layer liposomes, that is, liposomes containing only one bilayer.
A phospholipid is a lipid (fatty or fatlike) substance containing phosphorus in the
form of phosphoric acid, which fimctions as a structural component of a membrane.
The main lipid part is hydrophobic, and the phosphoryl or phosphate part
is hydrophilic. The hydration or uptake of water by phospholipids causes them
to spontaneously self-assemble into unilaminar liposomes. Mechanical agitation of
these unilaminar liposomes can convert them to multilayer liposomes that consist of
concentric bilayers. Unilaminar liposomes have diameters from the nanometer to the
micrometer ranges, with bilayers that are 5-10nm thick. Proteins can be incorpo-
rated into unilaminar liposomes to study their hnction in an environment resembling
that of their state in phospholipid bilayers of a living cell.
If polymerizable surfactants are employed, such as those containing acrylate,
acrylamido, allyl (CH2=CHCH2-), diallyl, methacrylate, or vinyl (CH2=CH-)
groups, then polymerization interactions can be carried out. When vesicles are
involved, the characteristic time for the polymerization is generally shorter than the
vesicle lifetime, so the final polymer is one that would be expected from the
monomers or precursors associated with the surfactant. However, when micelles are
involved, their lifetimes are generally short compared to the characteristic times of
the polymerization processes, and as a result the final product may differ consider-
ably from the starting materials. Surfactants with highly reactive polymerizable
groups such as acrylamide or styryl have been found to produce polymers with
molecular weights in excess of a million daltons. Those with polymerizable groups
of low reactivity such as allyl produce much smaller products, namely, products with
degrees of polymerization that can bring them close to the micelle size range prior to
polymerization.
Micelles and bilayers or liposomes have a number of applications in chemistry
and biology. Micelles can assist soap solutions to disperse insoluble organic
compounds, and permit them to be cleaned from surfaces. Micelles play a similar
role in digestion by permitting components of fat such as fatty acids, phospholipids,
cholesterol, and several vitamins (A, D, E, and K) to become soluble in water, and
thereby more easily processed by the digestive system. Liposomes can enclose
enzymes, and at the appropriate time they can break open and release the enzyme so
that it can perform its hction, such as catalyzing digestive processes.
Many biological membranes such as the plasma membrane of a red blood cell or
erythrocyte are composed of proteins and lipids, where a lipid is a fat or fatlike